Commercially available pearl-luster pigments which use thin-platelet like mica substrates coated with highly refractive metal oxides (called “interference color layer” hereinafter) such as titanium oxide etc. to exhibit color through interference are well known. However, since the mica is semi-transparent, their reflectivity is low; therefore, expression of interference color by action of interference is not fully achieved. Thus there appear the interference colored pigments where metal which is opaque and has a high optical reflectance is used as thin-platelet like substrate and an interference color layer (such as titanium oxide, etc) is coated on its surface (forex, JP (A) Hei. 1-110568, JP (A) Hei. 2-669, etc). In these interference colored pigments, the interference color is layer indirectly coated by means of the so-called sol-gel method avoiding an aqueous water treatment, since thin-plate like metal substrate have a disadvantage that they can easily react with water and easily be oxidized.
Further, goniochromatic pigments (multi-chromatic interference colored pigments) has also been known, wherein on the surface of thin-platelet like metal substrate, the first coating layer is formed using hydrated silicon oxide (as an example of a substance having a low refractive index) by the so-called Sol-Gel method, then other metal oxide having a high refractive index is coated thereon by vapor phase reaction and repeated too give a lure charge (color travel effect) due to changes in viewing angle (ex. JP (A) Hei. 8-209024, JP (A) Hei. 8-302237, JP (A) Hei. 9-124971).
Moreover, in order to obtain a highly anti-corrosive layer (passivation layer), it has been known that there were a product which is treated with phosphoric acids, etc, (e.g. DE 19836810.0 etc.), a product which is treated with organic phosphoric acids (e.g. JP (A) Hei. 3-74472, JP (PCT) 2001-502375, etc.), a product which is treated only with silica (e.g. JP (A) Hei. 8-209025; U.S. Pat. No. 2,885,366 and U.S. Pat. No. 3,954,496; etc.), a product where a treatment with containing volatile phosphorus compounds and volatile nitrogen containing organosilicon compounds by the vapor phase method is carried out (JP (A) Hei. 7-292279), and so forth.
However, in these conventional methods there is the disadvantage that expensive raw materials have to be used or that the inherent smoothness of the surface of thin-platelet like metal substrate is not maintained but is deteriorated or the dispensability of particles is insufficient resulting in irregular reflection whereby the reflected light on the surface of thin-platelet metal can not be adequately utilized. Accordingly, even when the upper interference color layer is further coated thereon it is not possible to sufficiently achieve the expression of the interference color.
In particular, because the anti-corrosive property to acidic aqueous solution is insufficient, it is not possible to carry out the treatment in an aqueous system where the cost is low and the operation is easy in terms of facilities thereon. Namely, it is not possible to adopt a method in which hydrated metal oxides obtained by neutral hydrolysis using water-soluble metal salts and alkali or by a thermal hydrolysis of water-soluble metal salts in an aqueous system, are coated, filtered, dried and, if desired, calcined (this will be defined as a wet-process method” as a whole including the description in the claims). In addition, as mentioned above, smoothness and denseness of the surface to which the anti-corrosive layer is coated are not sufficient and, further the affinity between the hydrated metal oxide layers constituting such an anti-corrosive layer and the hydrated metal oxide layer constituting the interference color layers coated thereon is not sufficient, whereby it is impossible to achieve a homogenity and denseness of the hydrated metal oxide layer constituting the interference color layer; whereby, interference color having a high coloration can not be exhibited.
In other cases such as the adoption of vapor phase method (e.g. in JP (A) Hei. 8-209024, JP (A) Hei. 8-302237, JP (A) Hei. 9-124971, etc.), there are disadvantage that production facilities are expensive, volatilizing temperature of the method used in restricted (JP(A)7-292279), whereby in general more strict reaction control is required than an aqueous phase reaction, etc. Thus there has been a demand for utilizing the above-mentioned conventional method which is so-called wet-process method.